Described as spare repair because I am not a gas man and no idea how it works. I put battery in lights up all stages one after next one and makes all the noises until finishing all the individual lights. Note battery socket back plastic was broken I fixed it professionally and  it looks stronger than original one please look at the one of the picture i was pointing it. As far as I can see it looks good operational, 

NOTE: Listed as spare repair, it does not mean faulty. I repair electronics and fully tested this gas tester, electronically it is fully functional, just don't know about testing gas because I am not a gas engineer and also I watched the company website for this item, it's functionality, it is all correct. This item is battery operated and they don't normally go wrong unless physically misused.